This project is aimed at determining the modes of transmission of two forms of leishmaniasis. Recently, in a control program to eliminate infected dogs, despite the marked decrease in canine leishmaniasis in the city, there was a rise in the number of human cases. We propose to determine whether persons with visceral leishmaniasis (VL) can transmit the causative L. chagasi to the sand fly vector Lu. longipalpis frequently enough to serve as a source for infection. To do this, we will determine: a. the frequency that L. longipalpis become infected with L. chagasi when biting patients with acute visceral leishmaniasis and asymptomatic persons with immunologic evidence of L. chagasi infections. b. whether sand flies infected with L. chagasi from humans can infect hamsters, thus showing they contain the infective form. Secondly, our studies on cutaneous leishmaniasis demonstrated clustering of cases involving persons of all age groups and both sexes implying that transmission was closer to houses than to places of work, and suggested that dogs (or humans) might be more important in transmission to humans than rodents (most of which were uninfected). We aim to: c. Confirm the presence of L.b. braziliensis in dogs, and evaluate the ability of dogs to infect sand flies and pass on the infection to hamsters. d. As with VL, ascertain whether humans can infect sand flies with L. braziliensis and thus play a role in transmission. e. Determine the host preference of Lu. whitmani and other native sand flies by the analysis of sand fly gut contents using PCR to distinguish human, canine, and other mammalian blood. f. To determine if the clustering of cases seen in households is due, in part, to genetic factors, we will type families for HLA class II. This project ties in directly with the goals of this ICIDR to decrease the morbidity and mortality of these important tropical diseases.